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| United States Patent |
5,137,851
|
|
Hormadaly
, et al.
|
August 11, 1992
|
Encapsulant composition
Abstract
A crystallizable glass composition comprising PbO and/or Bi.sub.2 O.sub.3,
ZnO, B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, SnO.sub.2 and optionally SiO.sub.2
and/or Al.sub.2 O.sub.3, having an optical density parameter .gtoreq.1.6.
| Inventors:
|
Hormadaly; Jacob (Wilmington, DE);
Taylor; Barry E. (Tokyo, JP)
|
| Assignee:
|
E. I. Du Pont de Nemours and Company (Wilmington, DE)
|
| Appl. No.:
|
714223 |
| Filed:
|
June 11, 1991 |
| Current U.S. Class: |
501/76; 501/10; 501/20 |
| Intern'l Class: |
C03C 003/074 |
| Field of Search: |
501/76,20,10
|
References Cited
U.S. Patent Documents
| 3088835 | May., 1963 | Pirooz | 501/76.
|
| 3113878 | Dec., 1963 | Martin | 501/5.
|
| 3959543 | May., 1976 | Ellis | 106/53.
|
| 3973975 | Aug., 1976 | Francel et al. | 501/76.
|
| 4824809 | Apr., 1989 | Grabowski et al. | 501/76.
|
| Foreign Patent Documents |
| 52-154825 | Jun., 1976 | JP.
| |
Primary Examiner: Bell; Mark L.
Assistant Examiner: Jones; Deborah
Claims
We claim:
1. A low melting crystallizable glass consisting essentially by weight of
30-40% PbO, Bi.sub.2 O.sub.3 or mixtures thereof, 35-50% ZnO, 10-30%
B.sub.2 O.sub.3, 0.5-3% chromium oxide, 0.5-10% SnO.sub.2, 0.5-10%
SiO.sub.2 and 0-10% Al.sub.2 O.sub.3, the ratio of Cr.sup.6+ to Cr.sup.3+
in the chromium oxide being sufficient that the optical density parameter
of the glass is at least 1.6.
2. The composition of claim 1 which contains 35-42% ZnO.
3. The composition of claim 1 which contains 35-40% PbO, Bi.sub.2 O.sub.3
or mixtures thereof.
4. The composition of claim 1 which contains 14-25% B.sub.2 O.sub.3.
5. A thick film composition comprising finely divided particles of the
glass composition of claim 1 dispersed in an organic medium comprising a
solution of organic polymer in nonvolatile solvent.
Description
FIELD OF INVENTION
The invention relates to encapsulant compositions. In particular, the
invention relates to low melting glass compositions which are suitable for
use as encapsulants for electronic circuits.
BACKGROUND OF THE INVENTION
Hybrid circuits should be encapsulated to insure resistor durability in
humid atmospheres. Furthermore, manufacturers prefer glass encapsulation
to protect the conductor metals from long term corrosion.
The encapsulant system must exhibit several features which are difficult to
achieve together. It must form a bubble-free seal at low enough firing
temperature and prevent shift of the underlying resistors. If the glass
flows too much, it will diffuse into the resistor and shift the value
upward. If it does not flow enough, it will not seal. The organic vehicle
necessary for screen printing must burn out at this low temperature. Thus
an ideal encapsulant should screen print smoothly and rapidly with a
vehicle which is decomposable at a low enough temperature to allow the
glass to flow sufficiently to form a seal, but not so much as to shift the
resistor.
Various glasses having low glass transition temperature (Tg) have been used
extensively as encapsulants for electronic circuits. These glasses usually
have had a high Temperature Coefficient of Expansion (TCE) which, unless
it is carefully matched to the adjacent circuit layers, can set up
substantial mechanical stresses which can lead to system failures.
An encapsulant, among its other functions, provides protection from the
environments to the underlying electronic circuit. To fulfill this
function the encapsulant should have sufficient durability to survive the
environments encountered in the production and the daily use of the
electronic circuits. Most low softening point glasses (referred to here as
"low melting glasses") have poor durability in acids and bases and their
durability tends to degrade as the glass transition temperature (Tg)
becomes lower. Although the majority of electronic circuits are not
expected to be used in very acidic or basic environments, some are exposed
to water and basic or acidic environments during the production. The final
stage in some fabrication processes involves an additional encapsulation
by an organic polymer, e.g., an epoxy. Some epoxy resins contain an amine
which can exert basic environment in humid atmosphere. Therefore,
durabilities in boiling water and basic solutions [triethanolamine (TEA),
in water to simulate amines in epoxy] are detailed here.
To combat this problem, a glass has been suggested by Asahi Glass KK in JPA
52/154825, which is a crystallizable zinc-lead-borate type glass that
undergoes crystallization when it is fired at 540.degree.-560.degree. C.
and produces a crystallized overlay having a low TCE. Although the glass
forms a dense overlay when fired at 540.degree. C., the layer tends to be
porous because of insufficient flow of the vitreous phase and excessive
crystallization. It is, of course, desirable to be able to fire at a
temperature in the 510.degree.-560.degree. C. range in order to avoid
interaction of the glass with the underlying circuit components during the
firing cycle. Therefore, there remains a real need for an encapsulating
glass which (1) can be fired in the 510.degree.-560.degree. C. range to
form a dense overlay, and (2) will form a dense overlay having good
encapsulating properties.
SUMMARY OF THE INVENTION
The invention is therefore directed primarily to a crystallizable glass
which is suitable as an encapsulant for silver conductive circuits
consisting essentially by weight of 30-40% PbO, Bi.sub.2 O.sub.3 or
mixtures thereof, 35-50% ZnO, 10-30% B.sub.2 O.sub.3, 0.5-3% Cr.sub.2
O.sub.3, 0.5-10% SiO.sub.2, SnO.sub.2 or mixtures and 0-10% Al.sub.2
O.sub.3, the optical density parameter of the glass being at least 1.6.
In a second aspect, the invention is directed to thick film pastes which
are dispersions of the above-described glass in an organic medium.
In a third aspect, the invention is directed to a method for encapsulating
resistors comprising the sequential steps of:
(1) forming a resistor on an electrically non-conductive substrate by
applying to the substrate a pattern of thick film resistor paste
comprising finely divided particles of resistive conductor material and
inorganic binder dispersed in organic medium and firing the patterned
paste to effect volatilization of the organic medium and sintering of the
inorganic binder; and
(2) completely covering the fired resistor pattern with a layer of the
above-described encapsulant thick film paste and firing the thick film
paste to effect volatilization of the organic medium and sintering of the
glass.
PRIOR ART
The closest prior art of which applicant is aware at the time of filing
this application is the following:
U.S. Pat. No. 3,088,835, Pirooz--The Pirooz patent is directed to a
crystallizable sealing glass composition containing 38-42% ZnO, 10-14%
SiO.sub.2, 11-29% wt. PbO, 9-13% copper oxide, and up to 5% wt. of other
glass forming oxides such as B.sub.2 O.sub.3. The copper oxide is
indicated to be essential for the purpose of adjusting the temperature
coefficient of expansion of the composition.
U.S. Pat. No. 3,113,878, Martin--The Martin patent is directed to a
crystallizable zinc silicoborate glass consisting of 60-70% ZnO, 19-25%
B.sub.2 O.sub.3 and 10-16% SiO.sub.2 and optionally "minor amounts" of
glass forming materials such as PbO, As.sub.2 O.sub.3 and Sb.sub.2 O.sub.3
and colorants. In particular, a glass is disclosed containing 60% ZnO,
22.5% B.sub.2 O.sub.3, 12.5% SiO.sub.2 and 5% PbO (Example 7). The glass
is disclosed to be useful as a sealing glass for preformed materials
having a Temperature Coefficient of Expansion (TCE) of
30-50.times.10.sup.-7.
Japanese Kokai 61/6018, assigned to Asahi Glass--The patent is directed to
a crystalline glass having a low melting point with the following
composition by weight: 31-40% PbO, 35-50% ZnO, 10-20% B.sub.2 O.sub.3,
2-6% SiO.sub.2, 0-3% SnO.sub.2 and 0-4% TiO.sub.2. The glass is completely
crystallized within 30 minutes upon heating to 500.degree.-600.degree. C.
The crystallized product has a TCE of 43-55.times.10.sup.-7 per .degree.
C. The glass is disclosed to be useful as a cover for thick film circuits.
It is disclosed to be applied as a paste by printing or brushing.
DETAILED DESCRIPTION OF THE INVENTION
Because the encapsulant composition of the invention is used with fired
resistors, it is necessary that the glass component be fired at a
relatively low temperature such that the glass will incur only a minimum
amount of diffusion into the resistor structure thereby minimizing
interaction with the resistor. Thus the glass component of the encapsulant
composition of the invention has been designed for use at a firing
temperature of about 530.degree.-580.degree. C.
It has been found that complete crystallization of a separate phase
throughout the encapsulated mass is not necessary. It is necessary only
that crystallization take place at the interface of the fired resistor and
the encapsulant layer in order to minimize glass flow at the interface.
The crystallized glass phase has been determined to be a mixture of
PbZn.sub.2 B.sub.2 O.sub.6, Zn.sub.2 SnO.sub.4, ZnSnO.sub.3 and Zn.sub.2
SiO.sub.4, of which the PbZn.sub.2 B.sub.2 O.sub.6 is the major component.
The crystallized glass has a different composition than both the parent
glass (or glasses) and the remainder glass.
The composition of the invention is required to contain by weight at least
35% ZnO, but not more than 50%. If less than 35% ZnO is used, the
composition will not crystallize sufficiently and the TCE is too high. On
the other hand, if more than 50% ZnO is used, the amount of
crystallization upon firing at 530.degree.-580.degree. C. is excessive. It
is preferred that the ZnO be present within the range of 35-45%.
In the practice of the invention, PbO and Bi.sub.2 O.sub.3 may be used
interchangeably. That is, either may be used to the exclusion of the
other, or both may be used together in all proportions. The PbO/Bi.sub.2
O.sub.3 must be present in the composition of the invention in an amount
of at least 30% but not more than 40%. If less than 31% PbO/Bi.sub.2
O.sub.3 is used, the TCE of the glass is too high and the softening point
of the glass becomes too low. It is preferred that the PbO/Bi.sub.2
O.sub.3 be present in the glass within the range of 35-40%.
The B.sub.2 O.sub.3 component is contained in the glass of the invention in
amounts ranging from 10-30% by weight. The B.sub.2 O.sub.3 serves an
important function in the glass in that contributes significantly to the
durability of the encapsulant layer. However, if more than 30% B.sub.2
O.sub.3 is used, the TCE of the composition tends to become too high. On
the other hand, if less than 10% B.sub.2 O.sub.3 is used, the degree of
crystallization during firing at 510.degree.-560.degree. C. tends to
become excessive. It is preferred that the B.sub.2 O.sub.3 be used within
the range of 10-18% by weight.
When low melting glasses or encapsulants are applied onto silver bearing
conductors, e.g., pure silver, silver platinum and silver palladium, and
processed in the conveyor furnace, they form a colored area above the
conductor. The color ranges from light yellow to brown depending on the
glass and conductor composition and processing conditions. Colored or
stained areas probably arise from the dissolution of silver in the glass
and subsequent precipitation of metallic silver during the processing.
Stain formation is a cosmetic defect which circuit manufacturers prefer to
minimize to the extent possible.
In order that the glasses of the invention may be stain-free when they are
used to encapsulate silver-containing conductor systems, it is necessary
to use at least 0.1% Cr.sub.2 O.sub.3 in the glass. However, it is
preferred not to use more than 3% Cr.sub.2 O.sub.3 lest the presence of
Cr.sup.6+ ions in the composition deteriorate the physical properties of
the glass excessively. As used herein, the term "stain-free glass" refers
to a glass which, when used as the inorganic phase in an encapsulant for
thick film silver-containing conductive layers which have been fired at
530.degree.-580.degree. C. results in the insulative layer's having a
natural green color. This is in contrast to stained glasses which have a
brown silver stain.
In order for the chromium oxide to be effective to reduce staining, it has
been found that it must be present in the glass in an oxidized state
whereby the Optical Density Parameter of the glass (f) is at least 1.6 and
preferably in the range of 2-3.5. To accomplish this, it is necessary that
the ratio of Cr6+ to Cr3+ be sufficiently high. This ratio is most readily
adjusted by melting the glass under oxidizing conditions whereby the
amount of Cr6+ is kept high with respect to the amount of Cr3+. This is
best accomplished during the glass melting process by bubbling air through
the melt.
As used herein, the term "Optical Density Parameter" (f) refers to the
ratio of the band absorption of the glass at 600 nm to the band absorption
at 400 nm as calculated from a diffused reflectance spectra (DRS) of the
glass under consideration. This measurement is discussed in greater detail
hereinbelow.
Both the SiO.sub.2 and SnO.sub.2 are important in the composition of the
invention because of their contribution to the durability (insolubility
and hermeticity) of the glass. At least 0.5 of each must be used in order
to get any significant technical effect. However, in order to avoid making
the softening point of the glass too high, the total amount of SiO.sub.2
and SnO.sub.2 must not exceed 10% and the amount of SnO.sub.2 must not
exceed 10%. It is preferred that both the SiO.sub.2 and SnO.sub.2 be
present within the composition of the invention within the range of 1-8%.
In addition to the above-described essential components, the composition
may optionally contain up to 5% by weight Al.sub.2 O.sub.3. Small amounts
of Al.sub.2 O.sub.3 are added to facilitate glass formation when the glass
is synthesized.
The surface area of the glass is not critical but is preferably in the
range of 0.75-4 m.sup.2 /g. Assuming a density of approximately 3-4
g/cm.sup.2, this range corresponds to an approximate particle size range
of 0.5-1 micron. A surface area of 1.5 m.sup.2 /g (approx. 1.3 micron) can
also be utilized. The preparation of such glass frits is well known and
consists, for example, in melting together the constituents of the glass
in the form of the oxides of the constitutents and pouring such molten
composition into water to form the frit. The batch ingredients may, of
course, be any compound that will yield the desired oxides under the usual
conditions of frit production. For example, boric oxide will be obtained
from boric acid, silicon dioxide will be produced from flint, zinc oxide
will be produced from zinc carbonate, etc. The glass is preferably milled
in a ball mill with water to reduce the particle size of the frit and to
obtain a frit of substantially uniform size.
The glasses of the invention are made by conventional glassmaking
techniques in that they are prepared by mixing the several metal oxide
components, heating the mixture to form a melt, forming a frit from the
melt by quenching in cold water and milling the frit to adjust the
particle size of the resulting glass powder. However, the process for
making these particular glasses is unconventional in two respects: (1) it
is necessary for the reasons discussed hereinabove to conduct the melting
step under oxidative conditions; and (2) it has been found to be necessary
to ball mill the frit.
As previously mentioned, to melt the admixture of metal oxides oxidatively
is readily accomplished by bubbling air through the molten mixture of
oxides which facilitates the formation of hexavalent chromium species. As
is well known in the art, heating is conducted to a peak temperature and
for a time such that the melt becomes entirely liquid and homogeneous. In
the present work, the components are premixed by shaking in a polyethylene
jar with plastic balls and then melted in a platinum crucible at the
desired temperature.
The melt is heated at the peak temperature for a period of 11/2 hours. The
melt is then poured into cold water. The maximum temperature of the water
during quenching is kept as low as possible by increasing the volume of
water to melt ratio. The crude frit after separation from water is freed
from residual water by drying in air or by displacing the water by rinsing
with methanol. The crude frit is then ball milled in water for 3-24 hours
in alumina containers using alumina balls.
After discharging the milled frit slurry, excess solvent is removed by
decantation and the frit powder is air dried at room temperature. The
dried powder is then screened through a 325-mesh screen to remove any
large particles.
There are today two principal ways of size reducing glass frits--ball
milling and jet milling. In the former, the grinding action is carried out
by inert ceramic balls. In the latter, the grinding action is carried out
by impingement of the frit particles in a high velocity stream. Both are
widely used, and each is ordinarily considered to be a suitable
alternative to the other. It is, however, an unusual characteristic of the
glasses of the invention that they perform best when they are wet ball
milled, rather than when they are jet milled. In particular, it has been
found that the glasses of the invention have reduced tendency to form
blisters upon firing over conductive circuits when they are wet ball
milled. The preferred milling medium is alumina balls. This unusual
phenomenon can be observed from the data given in Table 1.
Is is preferred that the encapsulant glass compositions of the invention
contain only the metal oxides discussed hereinabove. Nevertheless, it is
recognized that small amounts, up to 5% wt., of other glass modifying
oxides such as alkali metal oxides and alkaline earths can be added to the
encapsulant compositions without changing their essential character.
ORGANIC MEDIUM
Organic medium suitable for use in the invention are selected according to
the physical form in which the encapsulant compositions are applied. In
particular, the encapsulant glass frit can be applied as a thick film
paste by screen printing.
When the encapsulant is applied by screen printing, the particles thereof
are mixed with an inert liquid medium (vehicle) by mechanical mixing
(e.g., on a roll mill) to form a pastelike composition having suitable
consistency and rheology for screen printing. The latter is printed as a
"thick film" in the conventional manner.
The main purpose of the organic medium is to serve as a vehicle for
dispersion of the finely divided solids of the composition in such form
that it can readily be applied to ceramic or other substrates. Thus the
organic medium must first of all be one in which the solids are
dispersible with an adequate degree of stability. Secondly, the
rheological properties of the organic medium must be such that they lend
good application properties to the dispersion.
Most thick film compositions are applied to a substrate by means of screen
printing. Therefore, they must have appropriate viscosity so that they can
be passed through the screen readily. In addition, they should be
thixotropic in order that they set up rapidly after being screened,
thereby giving good resolution. While the rheological properties are of
primary importance, the organic medium is preferably formulated also to
give appropriate wettability of the solids and the substrate, good drying
rate, dried film strength sufficient to withstand rough handling and good
firing properties. Satisfactory appearance of the fired composition is
also important.
In view of all these criteria, a wide variety of liquids can be used as
organic medium. The organic medium for most thick film compositions is
typically a solution of resin in a solvent frequently also containing
thixotropic agents and wetting agents. The solvents usually boil within
the range of 130.degree.-350.degree. C.
Suitable solvents include kerosene, mineral spirits, dibutylphthalate,
butyl carbitol, butyl carbitol acetate, hexylene glycol and high boiling
alcohols and alcohol esters. Various combinations of these and other
solvents are formulated to obtain the desired viscosity and volatility.
By far the most frequently used and a frequently preferred resin for this
purpose is ethyl cellulose. However, resins such as ethylhydroxyethyl
cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins,
polymethacrylates of lower alcohols and monobutyl ether of ethylene glycol
monoacetate can also be used.
In the past, poly(alpha-methyl styrene) has been utilized as a resin for
thick film applications because of its excellent burn-out properties.
However, poly(alpha-methyl styrene) has not been widely used because thick
film pastes made therewith have exhibited very poor rheological
properties. It has, however, been found when the encapsulant composition
of the invention are formulated as thick film pastes using
poly(alpha-methyl styrene) dissolved in dibutyl carbitol, the resulting
paste has quite good rheological properties for screen printing. Thus, a
further suitable organic medium for formulating the encapsulant
composition of the invention as a thick film paste is a solution of 20-60%
wt. poly(alpha-methyl styrene) and 80-40% wt. dibutyl carbitol and
preferably 45-55% wt. poly(alpha-methyl styrene) and 55-45% wt. dibutyl
carbitol.
Among the thixotropic agents which are commonly used as hydrogenated castor
oil and derivatives thereof and ethyl cellulose. It is, of course, not
always necessary to incorporate a thixotropic agent since the solvent
resin properties coupled with the shear thinning inherent in any
suspension may alone be suitable in this regard. Suitable wetting agents
include phosphate esters and soya lecithin.
The ratio of organic medium to solids in the paste dispersions can vary
considerably and depends upon the manner in which the dispersion is to be
applied and the kind of organic medium used. Normally, to achieve good
coverage, the dispersions will contain complementally by weight 40-90%
solids and 60-10% organic medium.
The pastes are conveniently prepared on medium (vehicle) utilized is
determined mainly by the final desired formulation viscosity and print
thickness.
TEST PROCEDURES
Laser Trim Stability--Laser trimming of thick film resistors is an
important technique for the production of hybrid microelectronic circuits.
([A discussion can be found in Thick Film Hybrid Microcircuit Technology
by D. W. Hamer and J. V. Biggers (Wiley, 19072, p. 173 ff.] Its use can be
understood by considering that the resistances of a particular resistor
printed with the same resistive ink on a group of substrates has a
Gussian-like distribution. To make all the resistors have the same design
value for proper circuit performance, a laser is used to trim resistances
up by removing (vaporizing) a small portion of the resistor material. The
stability of the trimmed resistor is then a measure of the fractional
change in resistance that occurs after laser trimming. Low resistance
change--high stability--is necessary so that the resistance remains close
to its design value for proper circuit performance.
Shift on Aging at 150.degree. C.--After initial measurement of resistance
at room temperature, the resistor is placed into a heating cabinet at
150.degree. C. in dry air and held at that temperature for a specified
time (usually 1,000 hours). At the end of the specified time, the resistor
is removed and allowed to cool to room temperature. The resistance is
again measured and the change in resistance calculated by comparison with
the initial resistance measurement.
Hermeticity--This test is performed in the same manner as the preceding
Shift on Aging Test, except that the air within the heating cabinet is
maintained at 85% relative humidity (RH) at 85.degree. C. (85%
RH/85.degree. C.).
Resistance Measurement and Calculations--The test substrates are mounted on
terminal posts within a controlled temperature chamber and electrically
connected to a digital ohm-meter. The temperature in the chamber is
adjusted and allowed to equilibrate, after which the resistance of the
test resistor on each substrate is measured and recorded.
Silver Migration Resistance Test--The following procedure is used to test
the compositions of the invention with respect to their capability to
resist silver migration:
1. A series of parallel thick film silver conductor lines is printed on a
96% Al.sub.2 O.sub.3 substrate using a 325-mesh screen having 1.1 mil
wire. The conductor pattern is then fired.
2. An overglaze strip is printed over the conductor lines substrate using a
200-mesh screen having 1.6 mil wire. The patterned overglaze paste is
fired at 500.degree.-600.degree. C. using a short 20-30 minutes firing
cycle.
3. A drop of deionized water is placed on the fired overglaze between the
conductor lines and a 20 volt DC current is applied for 15 minutes.
4. After applying current for 15 minutes, the assembly is examined under a
microscope and examined visually. If any interaction is observed such as
bubbles, staining or dendrite formation, the assembly is deemed to have
failed the test, which is then terminated.
The above test is based upon procedures described by S. J. Krumbein in his
article entitled Metallic Electromigration Phenomena in IEEE Transactions
on Components, Hybrids and Manufacturing Technology, Vol. II, No. 1, March
1988.
Durability was measured as follows: Weighed 1.times.1 inch alumina
substrates were screen printed with the desired overglaze, dried and
subsequently fired at 560.degree. C. peak temperature in a belt furnace.
The fired part is then weighed again to record the net weight of the
overglaze, after which it is subjected to boiling water for 5 hrs. or
1.49% TEA for 24 hrs. at room temperature (25 grams of 1.49% TEA solution
were used for each 1.times.1 substrate, 50 g of distilled water were used
for each 1.times.1 substrate in the boiling water test). After exposure to
the test solution, the parts were rinsed with distilled water and dried in
oven 120.degree. C..+-.10.degree. C. for .about.16 hrs. Weights were
recorded again to determine weight loss (.DELTA.W). All weight
measurements were done on an analytical balance .+-.0.0001 g accuracy, so
the accuracy in the measured .DELTA.W is .+-.0.0003 g. Durability in
boiling water for hours is outstanding for all compositions measured
(Table I). Weight loss ranges were from 0.0001 g to 0.0005 g which is
within experimental error. Durability in 1.49% TEA solution is given
below.
Diffuse Reflectance Spectra--Cr-containing glasses tend to reach an
equilibrium during the melting. The ratio of Cr(VI)/Cr(III) varies with
temperature, melting time, glass composition and the oxidizing or reducing
conditions during the melting. Cr(III)-Cr(VI) equilibrium in binary alkali
silicate.sup.(1), UV absorption of binary borate containing Cr(VI).sup.(2)
and the UV absorption of Cr(VI) in binary and ternary alkali and alkaline
earth borates.sup.(3) were reported in the literature. Literature data
show that Cr(VI) in ternary borate glasses exhibits two intense absorption
bands in the UV: one at 250-270 nm and the second at 350-370 nm. These
bands are similar to the absorptions of solutions containing CrO4= (basic)
and dichromate (acidic) species. Cr(III) absorption in glasses is similar
to its absorption in solution, i.e., two weak bands in the visible
.about.440 nm and .about.600 nm.
.sup.(1) P. Nash & R. W. Douglas, Phy. Chem. Glasses 6 (6), 197 (1965).
.sup.(2) A. Paul & R. W. Douglas, Phy. Chem. Glasses 8 (4), 151 (1967).
.sup.(3) A. Paul & R. W. Douglas, Phy. Chem. Glasses 9 (1), 27 (1968).
The glass of the invention does not lend itself to a simple absorption
study because it is a crystallizable glass and has strong absorption in
the UV due to the allowed transitions of Pb.sup.+2. These transitions will
mask some of the absorption bands of Cr(VI). These features do not allow
quantitative estimation of Cr(VI) by UV-Vis absorption.
Diffused reflectance spectra were obtained for various glasses of the
invention and similar Cr-free glass. Diffused reflectance spectra is
basically a qualitative tool when comparing various samples. Analysis of
the spectra shows that one can define a pure number-ratio of two
absorption bands, which is proportional to Cr(IV)/Cr(III) ratio, thus
facilitating correlation between meltings parameters and diffuse
absorption spectra.
The absorption of band at .about.600 nm is due to Cr(III) only, while the
absorption band at 440 nm is due to Cr(III) and Cr(VI). If we denote the
optical density at 600 nm as D(.lambda.2) and the optical density at 440
nm as D.lambda.(1) then:
##EQU1##
where .alpha., .alpha.' and .beta. are constants related to the specific
absorption bands of Cr(III) and Cr(VI).
The ratio .function. is directly related to [Cr(VI)/[Cr(III)] ratio; by
this analysis we have a quantitative parameter to compare sample to sample
and to relate this parameter (.function.) to the melting conditions. More
information is possible by using .alpha., .alpha.' and .beta. from the
literature data.
The parameter .function. was calculated from the diffuse reflectance
spectra for glasses of the invention melted under various conditions. The
error in the .function. values is .+-.0.1.
EXAMPLES
Examples 1-18
A series of 18 glasses having the same composition of metal oxides was
prepared in different ways in which the following variables were studied:
crucible material, melting time and temperature, ball milling v. jet
milling, and oxidative conditions. Each of the glasses was formulated into
a thick film paste using as organic medium a solution of ethyl cellulose
in terpineols. Each of the pastes was then screen printed and fired at
560.degree. C. over a previously fired pattern of conductive thick film
paste in which the conductive phase was a silver-containing metal (Ag,
Ag/Pd, Ag/Pt). The composition by weight of the starting metal oxides in
the glass was as follows:
______________________________________
38.2% ZnO
38.3% Pb
17.2% B.sub.2 O.sub.3
1.2% Cr.sub.2 O.sub.3
2.9% SnO.sub.2
2.3% SiO.sub.2
______________________________________
The melting conditions and properties of the glasses are given in Table 1
below:
TABLE 1
______________________________________
Effect of Glass Processing Conditions
Ex-
am- Crucible Mill- Blister-
ple Materials
Melt Conditions ing f ing
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1 Pt No bubbler tube. Ball 1.49 No
2 Pt Bubbler tube, 70 min.
Jet 2.13 Yes
3 Pt Bubbler tube, 70 min.
Ball 2.10 No
4 Pt Bubbler tube; 30 min.,
Jet 1.79 Yes
1000.degree. C.
5 Pt Bubbler tube, 30 min.,
Ball 1.79 No
1000.degree. C.
6 Pt Bubbler tube, 30 min.,
Jet 1.75 Yes
800.degree. C.
30 min., 1000.degree. C.
7 Pt Bubbler tube, 30 min.,
Ball 1.73 No
800.degree. C.
30 min., 1000.degree. C.
8 Pt Plant Scale, bubbler tube,
Jet 2.31 Yes
30 min., 1000.degree. C.
9 Kyanite O.sub.2 from Pb.sub.3 O.sub.4
Ball 3.26 No
decomposition
60 min. at 500.degree. C.
60 min. at 1000.degree. C.
10 Kyanite O.sub.2 from Pb.sub.3 O.sub.4
Ball 3.10 No
decomposition
60 min. at 500.degree. C.
60 min at 1000.degree. C.
11 Kyanite O.sub.2 from Pb.sub.3 O.sub.4
Ball 3.21 No
decomposition
60 min. at 500.degree. C.
60 min. at 1000.degree. C.
12 Pt Bubbler tube, 70 min.,
Ball 2.03 No
1000.degree. C.
13 Pt Bubbler tube, 20 min.,
Ball 1.83 No
1000.degree. C.
14 Pt Bubbler tube, 30 min.,
Ball 2.04 No
1000.degree. C.
15 Pt Bubbler tube, 40 min.,
Ball 2.25 No
1000.degree. C.
16 Pt Bubbler tube, 50 min.,
Ball 1.68 No
1000.degree. C.
17 Pt Bubbler tube, 60 min.,
Ball 1.76 No
1000.degree. C.
18 Pt Bubbler tube, 70 min.,
Ball 1.97 No
1000.degree. C.
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The above data show that the use of oxidative melting conditions is
effective to raise the optical density parameter of the glasses of the
invention and that the use of ball milling in place of jet milling is
effective in eliminating blistering of the encapsulant layers made from
these glasses by firing the printed thick films.
In the foregoing examples, the bubbling rate was controlled and was the
same for Examples 2-7 and 12-18. Example 8 was controlled at a higher
bubble rate.
Examples 19-23
When the glass of Examples 1-18 was tested as to durability, the results
were as follows:
TABLE 2
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Glass Durability
Example Test Time Avg. Wt. Loss
Temp.
No. Medium (hrs.) Loss (% Wt.)
(.degree.C.)
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19 Boiling H2O 5 1.0 100
20 0.1N NaOH 1 4.2 22
21 0.001N NaOH 1 None 22
22 1.5% wt. TEA
24 6.5 22
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Four lots of glass having the same composition as Examples 19-22 were
tested as to TCE. All were found to have a TCE of
57-60.times.10-7/.degree.C. When the preceding four glasses were used to
encapsulate a silver conductor, it was found after 15 minutes that none of
the encapsulant layers had bubbles, there was no dendrite formation and
there was no staining of the encapsulant glass.
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